1. Field of the Invention
This invention relates generally to digital data modems and more specifically to apparatus responsive to a random multilevel data signal for synchronizing the characteristics of such a modem with the characteristics of an input analog signal.
2. Description of the Prior Art
Multipoint communication networks typically include a master station having a transmitter and a receiver which communicates through a channel with a plurality of slave stations also having a transmitter and a receiver. In these networks, the channel is typically a telephone party line connecting the master station with all of the slave stations. The line between the master station and a specific slave station has individual characteristics which affect differently the signals transmitted to or from other slave stations.
The signal transmitted to usually an analog signal which is sampled to provide a random multilevel data signal in the receiver. The analog signal has timing, phase, and line characteristics which are preferably matched by the receiver in order to provide for the detection of the data in the incoming signal. If these characteristics of the receiver are not adequately synchronized with the characteristics of the input analog signal, the data detected by the receiver is inaccurate and therefore useless.
When a receiver falls out of synchronization with the incoming signal, it is desirable to restart or resynchronize the receiver. In some systems of the prior art, this has been accomplished by stopping the normal data transmission and substituting a predetermined, usually bilevel, data sequence. In response to this known data sequence, the receiver has determined the timing and phase error and adjusted the associated characteristics of the receiver to provide the desired synchronization.
Although these systems which provide for the transmission of a known data sequence can provide rapid synchronization, they may not be desirable in a particular multipoint communication network where it is not convenient to stop the flow of normal data. In a specific system, for example, a master station might communicate simultaneously with hundreds of slave stations to transmit data associated with a weather report. If the receiver at one of the slave stations were to fall out of synchronization with the incoming signal, it would clearly be undesirable to stop all data transmission merely to synchronize the one receiver. This would severely reduce the data throughput of the system since all of the receivers would have to wait for the one receiver to synchronize.